Photoelectric responsive ionic channel for sustainable energy harvesting

Access to sustainable energy is paramount in today’s world, with a significant emphasis on solar and water-based energy sources. Herein, we develop photo-responsive ionic dye-sensitized covalent organic framework membranes. These innovative membranes are designed to significantly enhance selective ion transport by exploiting the intricate interplay between photons, electrons, and ions. The nanofluidic devices engineered in our study showcase exceptional cation conductivity. Additionally, they can adeptly convert light into electrical signals due to photoexcitation-triggered ion movement. Combining the effects of salinity gradients with photo-induced ion movement, the efficiency of these devices is notably amplified. Specifically, under a salinity differential of 0.5/0.01 M NaCl and light exposure, the device reaches a peak power density of 129 W m−2, outperforming the current market standard by approximately 26-fold. Beyond introducing the idea of photoelectric activity in ionic membranes, our research highlights a potential pathway to cater to the escalating global energy needs.


REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): The manuscript described a new COF membrane structure with dye molecules integrated to form polar channels, presumably by microphase demixing.The system turns out to be permselective for cations and is able to turn osmotic gradients into electric energy, that is to generate "blue energy".When light is shone on the membrane, the current is increasing, thus reacting record high blues energy values which outperform current membranes by a factor of 30.Of course, I always doubt such high improvement rates, but there is nothing which makes me skeptic in the manuscript so I tend to trust the authors.For my naivety, I never understood how electric neutrality can be kept by letting only cations pass a membrane, so the voltages must be zero-current polarization voltages, while the current is coming from the depleting reference electrode, and I recommend to make one measurement without reference, as floating potential is no problem in energy generation.But I am sure that my missing education can be compensated by some explanatory sentences also picking up generalist readers like me.

Reviewer #2 (Remarks to the Author):
This manuscript reports a dye-encapsulated covalent-organic framework membrane that shows cation selectivity and can be used in photo-enhanced osmotic energy harvesting.Although a high output power density is achieved in this work, there are some problems with the experimental results and explanations.Therefore, this reviewer recommends a major revision before this manuscript is ready publication.Issues that need to be addressed are listed as follows.1.In the "Introduction" part (Page 3), it is described that the light absorption will change the redox potential of dye molecules.First, it needs to be further explained here; second, this phenomenon is not mentioned when the experiment results are discussed.Does it have anything to do with photo-enhanced osmotic energy harvesting? 2. The dense layer of PAN substrate is used to load the COF material.However, this dense layer itself is porous and can conduct ions.It seems that this layer will contribute to the ion currents measured in the experiment.In addition, COF can also grow inside this porous layer.More detailed characterization needs to be provided.3. The thickness of the COF layer in Fig. 2c is difficult to be distinguished.4. The size of the membrane ("approximately 1.5 and 2.0 mm") is rather small, and it does not seem to match the Fig. S22.How to handle such a small sample in the experiment? 5.The results presented in Fig 3b and Fig. 3d both give the permselectivity of the membrane, but there is a big difference in the results.An explanation needs to be given here.Keep the more reasonable one in the article.6.For Fig. 3c, the details of the calculation of the conductivity should be given.7. It is described that the membrane structure might be broken with high HB content.Evidence needs to be given here.8. For osmotic energy harvesting, some important information is missing, including the testing area, the method to obtain the redox potential of the Ag/AgCl electrode, the list of the redox potential under different salt gradient, the method to acquire the osmotic potential and osmotic current.These information is very important and must be described in the article.9.It is described that the system is stable in 24 days.Considering that the volume of the solution is very small, after 24-day ion diffusion from high-concentration side to the low-concentration side, will the ion concentration at the of the low-concentration side change?A simple estimate can be provided here.10.The inset of Fig. 4d seems to have nothing to do with the flexibility and foldability of the device.11.For photo-induced ion transport, more control experiments need to be provided, such as the influence of wavelength of the light (e.g., absorption wavelengths of dye molecules), the intensity of the light.In addition, if a long-strip membrane is used, how will the current and potential change with light irradiation at different parts.12.In this work, the light uniformly irradiates on the membrane, so why does it cause ionic flow?What is the direction of the ionic flow?Why is this direction the same as the direction of osmotic ionic flow? 13.The temperature change of the membrane under light irradiation can be characterized using an infrared camera.14.The HNMR and UV-Vis experiment is done with HB, which is not reasonable.The authors should use HB@COF materials.

Reviewer #1:
Comment 1: The manuscript described a new COF membrane structure with dye molecules integrated to form polar channels, presumably by microphase demixing.The system turns out to be permselective for cations and is able to turn osmotic gradients into electric energy, that is to generate "blue energy".When light is shone on the membrane, the current is increasing, thus reacting record high blues energy values which outperform current membranes by a factor of 30.Of course, I always doubt such high improvement rates, but there is nothing which makes me skeptic in the manuscript so I tend to trust the authors.For my naivety, I never understood how electric neutrality can be kept by letting only cations pass a membrane, so the voltages must be zero-current polarization voltages, while the current is coming from the depleting reference electrode, and I recommend to make one measurement without reference, as floating potential is no problem in energy generation.But I am sure that my missing education can be compensated by some explanatory sentences also picking up generalist readers like me.
Response: We appreciate the reviewer's comments and the support offered for the work conducted in our study.Concerning the issue raised by the reviewer regarding maintaining electric neutrality with only cations passing through the membrane, we have addressed this concern in our study.We integrated a pair of redox electrodes (Ag/AgCl) into the system to facilitate redox reactions that help balance the charge as cations selectively migrate across the membrane and accumulate in the low-salinity reservoir.This configuration ensures that anions remain in the high-salinity reservoir, thereby maintaining charge separation.To provide a clearer understanding of the working principle, we have included a schematic illustration in the revised manuscript.This illustration, found in Supplementary Fig. 1

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): I went through the modified manuscript and the given responses to the authors, and I am happy with both of them.This is why I am ready to propose the manuscript for publication as it is.I am sure it will create a lot of discussions within this blue energy community...

Reviewer #2 (Remarks to the Author):
The authors have carefully responded to the raised comments.However, the mechanism of light enhanced ion separation and potential generation needs to be further explained.
It is described that "PCET processes generate a built-in electric field across the membrane".
It is worth pointing out that the structure of the membrane device is different from either cell membrane or photovoltaic device.A cell membrane is of a thickness of only several nanometers, and a photovoltaic device consists of asymmetric layered materials.While, the membrane device used in this work is of millimeter scale and has a symmetric structure.The key is to give the reason of why the PCET happened at each small dye molecule can build an electric field inside the millimeter-scale membrane.A further explanation needs to be given at the molecular level.

Reviewer: 1
Comment 1: I went through the modified manuscript and the given responses to the authors, and I am happy with both of them.This is why I am ready to propose the manuscript for publication as it is.I am sure it will create a lot of discussions within this blue energy community.
Response: We appreciate the reviewer's comments and the support offered for the work conducted in our study.

Reviewer: 2
Comment 1: The authors have carefully responded to the raised comments.However, the mechanism of light enhanced ion separation and potential generation needs to be further explained.It is described that "PCET processes generate a built-in electric field across the membrane".It is worth pointing out that the structure of the membrane device is different from either cell membrane or photovoltaic device.A cell membrane is of a thickness of only several nanometers, and a photovoltaic device consists of asymmetric layered materials.While, the membrane device used in this work is of millimeter scale and has a symmetric structure.The key is to give the reason of why the PCET happened at each small dye molecule can build an electric field inside the millimeter-scale membrane.A further explanation needs to be given at the molecular level.We sincerely thank the reviewer for dedicating time to assess our manuscript and for their valuable constructive comments.
and cover-letter Figure 1 of both the revised manuscript and the accompanying cover letter, visually demonstrates how the redox electrodes enable the conversion of ionic charge flux into an electrical current while maintaining charge separation and electric neutrality.The redox reactions occurring at the electrodes not only address the concern of electric neutrality but also contribute to the efficient generation of blue energy.By connecting an external load to the system, we can harness the potential energy difference between the high-salinity and low-salinity reservoirs to generate electricity effectively.Cover-letter Figure 1.(a) Schematic illustration of harvesting energy from salt concentration gradient via the HBx@COF-301/PAN membrane.A selective membrane favors the transport of counterions from the highconcentration reservoir (left) to the low-concentration reservoir (right) and thus generates an ionic current.The negatively charged HBx@COF-301/PAN membranes, which favor the flux of cations over anions, are shown.The difference between the cationic flux and anionic flux is the ionic current, .The Ag/AgCl electrodes are necessary to convert the ionic current to electrical current via redox reactions, thereby closing the electrical circuit.(b) Equivalent circuit representing the harvesting of energy by an external load,   , where   is the internal resistance of the nanopore-based power generator and ∆  is the membrane potential.

Response:
photoelectric response is solely dependent on the illuminated area and remains unaffected by the specific location of the membrane.